The invention relates to the field of controlled rate freezers, and more particularly to the field of devices for freezing animal embryos.
It is currently practised in the area of breeding domestic livestock, such as cattle, to improve the genetic characteristics of a particular herd by obtaining an embryo from a high quality donor animal and transplanting the embryo to the uterus of a lower quality host animal which then gives birth to the higher quality young animal. This is a much less expensive way of obtaining the higher quality young animal than having the donor animal give birth to the young animal and then transporting the animal to the new herd.
In order to transport the embryo over any significant distance it is necessary to freeze the embryo and maintain it in frozen condition, typically stored in liquid nitrogen. The process of freezing the embryo is a delicate process due to the properties of living tissue. The rate at which the embryo is frozen is crucial. Cooling the embryo at too rapid a rate will result in the death of the embryo.
In the case of bovine embryos, a particular bovine embryo freezing curve has been found to be most effective. Starting at a temperature of approximately 20 degrees centigrade, the temperature of the embryo is allowed to drop guickly to about minus 5 degrees centigrade. The temperature of the embryo is held at that temperature for about five minutes while the solution containing the embryo is "seeded". This is the process of the induction of ice crystallization in the buffer solution in which the embryo is stored in the straw. Typically, it is a solution having about 10 percent qlycerol. Once the buffer solution is completely "seeded", the temperature of the embryo is dropped at a rate of about 0.5 degrees centigrade per minute to a temperature of between minus 30 degrees centigrade and minus 35 degrees centigrade. The embryo can then be plunged directly into liquid nitrogen (-196.degree. C.) for storage.
In the past, two general types of embryo freezers have been utilized. The first type has been a computer-controlled liquid nitrogen freezer. In this case, the embryo, stored in a straw or ampoule, is held in a temperature- controlled chamber. The temperature of the chamber is controlled by adjusting a flow of liquid nitrogen vapour to the chamber using a computer-controlled valve and also utilizing a heater in conjunction with the flow of liquid nitrogen. The computer is previously programmed with the proper freezing curve and the controller regulates the flow of liquid nitrogen and the heating action of the heater in order to follow the prescribed freezing curve. Such devices require relatively complicated electronic circuitry which makes the devices both expensive and bulky and subject to loss of calibration when transported. Also in some of these devices it is necessary to open the chamber at one point during each freezing operation. This can result in an unwanted rapid warming of the embryo and possible damage to the embryo.
A second common type of embryo freezer is a mechanical freon refrigeration device. In such devices, the standard freon mechanical refrigerator cools an alcohol "heat-sink" freezing chamber. The use of a heater in conjunction with the refrigerator allows the rate of temperature change to be regulated. This sort of device is preferable to the liquid nitrogen chamber in that the temperature of the embryo immersed in the alcohol heat-sink is less readily influenced by the opening and closing of the lid of the freezing chamber. However, again the device requires complicated electronic circuitry which makes it expensive, bulky, difficult to transport and difficult to maintain in calibration.
A recent improvement in such devices has been made by Dr. Peter Elsden. The lack of portability of the previous devices has been overcome by freezing the embryos in a simple solid metal cylinder which is suspended in the neck of a liquid nitrogen refrigerator tank. The solid metal cylinder is bored out to receive the straws. The height at which the cylinder is suspended in the neck determines the rate at which the metal core cools. The problem with the device, however, is the fact that there is a large temperature gradient between the top and bottom of the metal cylinder. For proper operation, all the embryos stored in the cylinder as well as the temperature sensing tip of the temperature probe must be suspended at exactly the same height within the metal cylinder. This requires considerable care when placing the embryos within their straws. Also, there will be an air space between the straw and the supporting metal of the freezing chamber and this makes the heat transfer which occurs within the freezing chamber less efficient than if the straw were immersed in a "heat-sink" liquid such as alcohol.